There is a wide variety of known DC converter topologies or configurations which are derived from at least one buck regulator and/or at least one boost regulator, with or without a transformer that may provide voltage transformation and/or electrical isolation between input and output circuits of the converter. As is known, without voltage transformation a buck regulator provides an output voltage which is lower than its input voltage, and a boost regulator provides an output voltage which is greater than its input voltage. Buck and boost regulators can be regarded as duals of one another; for example, with switches replacing diodes a buck converter can be operated in a reverse direction as a boost regulator.
One DC converter topology comprises a cascade of a buck regulator followed by a boost regulator, forming a buck-boost converter. An inductor of a buck-boost converter can be arranged as two windings of a transformer, forming a flyback converter which has a well known form. In a flyback converter, the transformer constituted by the inductive component of the converter can provide voltage transformation and/or electrical isolation between the input and output circuits of the converter.
Significant technical obstacles need to be overcome by DC converters having a large ratio, for example of about 10:1, between input and output voltages of the converter without using a transformer for voltage transformation. There is an increasing need for non-isolating converters providing such large voltage ratios, for example for providing low supply voltages (for example of the order of one volt) to power electronic circuits.
One of these obstacles is, typically, a need to drive the gate of one of the MOSFET switches of the converter at a voltage level above that of the converter input voltage. The flyback converter topology discussed above avoids this problem, but achieves this at the expense of higher peak currents, for example four times higher in discontinuous mode.
Another of these obstacles is a need to power a control circuit of the converter from the input voltage in order to start up the converter. This adds complexity to the control circuit, with a need for high voltage components to withstand the relatively (compared to the output voltage) high input voltage. Such components are not easy to incorporate in an integrated circuit form of the control circuit.
The non-isolating converter topology most commonly used to provide an output voltage lower than the input voltage is that of the buck regulator, typically comprising two MOSFET switches referred to as the high side and low side switches or MOSFETs. Both of these switches must be rated for the maximum input voltage and the maximum output or load current. With large input to output voltage ratios, these MOSFETs must be physically large to withstand voltage and current stresses. Consequently, these MOSFETs have high parasitic capacitances which must be driven for correct switching, and the resulting switching losses become the dominant contributor to loss of efficiency at high switching frequencies, for example above about 500 kHz. Achieving-switching speeds for correct switching of the converter at the very low duty cycles at which the buck regulator must operate also requires high peak gate drive currents, and significantly increases the impact of parasitic inductances at high switching frequencies.
Accordingly, there is a need to provide an improved DC converter in which some or all of these difficulties are reduced or avoided.